The invention relates to a hearing aid having a microphone, a receiver, and a signal processing unit with a circuit unit for producing constant sound pressure levels. The circuit unit has an envelope generator for producing an envelope value of a signal value to be processed and a divider for calculating a ratio of the signal value and the calculated envelope value of the signal value to be processed. The invention also relates to a method for operating a hearing aid wherein a microphone, a receiver, and a signal processing unit with a circuit unit for producing constant sound pressure levels are provided.
A common problem with hearing aids is to avoid exceeding an adjustable discomfort threshold in the signal amplification, and hence to produce constant sound pressure levels. This can be achieved by signal amplitude regulation with a feedback structure, the (amplified) input signal being fed to a level detector and the output value of the latter being delivered to a control loop with attenuation of the output signal at increasing input signal strength.
In the event of high amplification within the control loop, phase shifts of approximately 180xc2x0 occur so that undesired co-coupling takes place instead of the required anti-coupling.
WO 97/28600 discloses a xe2x80x9ccompanderxe2x80x9d, i.e. compressor/expander circuit, for use in a battery-driven instrument. The instrument comprises an input for picking up an analog input signal, a signal processing unit for producing a digital signal, which corresponds to the envelope curve of the input signal, and a combining unit for combining the digitized envelope curve with the analog input signal, in order to provide an analog output signal. When compressing the analog input signal, the output signal is proportional to the product of the input signal and the reciprocal of the envelope curve. When expanding the analog input signal, the output signal is proportional to the product of the input signal and the envelope curve.
U.S. Pat. No. 5,724,433 discloses a circuit for automatic gain control in a hearing aid. The circuit comprises an adaptive compression and filter circuit having a plurality of channels which are connected to a common output. The gain of a channel is dependent on the output signal and a discomfort threshold for the channel in question. The gain adjustment of the channel in question is increased whenever the output signal of the channel is below a pre-adjustable threshold, and is reduced whenever the output signal is above the threshold.
In another embodiment, a forward structure with a gain table is used to produce constant sound pressure levels in hearing aids according to the prior art. This has, inter alia, the disadvantage that the gain table grows super-proportionately with the stored gains at higher amplification. Further, discontinuities may occur in the transition range between two stored gains, which can lead to an increased perturbation factor of the signal.
It is an object of the invention to provide a hearing aid, as well as a method for operating a hearing aid, in which the sound pressure level at the output does not exceed an adjustable discomfort threshold.
The object is achieved, in relation to the hearing aid, wherein a microphone, a receiver and a signal processing unit with a circuit unit for producing constant sound pressure levels are provided. The circuit unit has an envelope generator for producing an envelope value of a signal value to be processed and a divider for calculating a ratio of the signal value and the calculated envelope value of the signal value to be processed. A comparator element is provided for comparing the envelope value from the envelope generator with a gain parameter. Further processing of the signal to be processed takes place based on the gain parameter when the envelope value falls below the gain parameter, and based on the envelope value when the envelope value reaches or exceeds the gain parameter. In relation to the method, the object is achieved by providing a microphone, a receiver and a signal processing unit with a circuit unit for producing constant sound pressure levels. The envelope value is compared with a gain parameter and the signal to be processed is processed further based on the envelope value if the gain parameter is reached or exceeded. Otherwise the signal to be processed is processed further based on the gain parameter.
A concept of the invention is that signal values of a signal, whose envelope values exceed an adjustable threshold value, are divided by the envelope values. This gives rise to a signal having a constant envelope. If this signal is output to the receiver of the hearing aid, then the resulting signal has a constant sound pressure level.
The invention is distinguished in that, in the case of a hearing aid, the sound pressure level at the output does not exceed an adjustable threshold value, and is kept constant over a large variation range of the sound pressure level at the input and/or a large amplification range. This is achieved at low cost and with at most minor signal distortions.
In the hearing aid according to the invention, the envelope of the signal which is to be processed, and whose sound pressure level is to be limited, is calculated in an envelope generator. Ratios of the signal values of the signal to be processed and the associated envelope values are calculated in a divider. The signal values at the output of the divider therefore lie in a restricted value range, for example in the range between xe2x88x921 and +1. Owing to the limited rise time of envelope generators that can be made, it is possible in practice for signal values to lie briefly outside the range enclosed by the envelope. According to one embodiment of the invention, such signal values are detected and likewise transposed into the restricted value range. For example, a constant value is allocated to each signal value outside the range enclosed by the envelope. The output signal comprised of the individual output signal values has a substantially constant envelope curve over a specific amplification range of the hearing aid.
The hearing aid according to the invention has, in particular, digital signal processing with a DSP element. In this case, the circuit complexity of the hearing aid according to the invention can be reduced in an advantageous embodiment, through detection of the sign of the signal and envelope values to be processed, and by processing only positive values and inverting negative values. By means of this, a 1-quadrant divider, which comprises a simply constructed shift-subtract unit, can be used as the divider.
In order to reduce the risk of clipping, the signal value of the signal to be processed is reduced by an adjustable corrector element before taking the ratio.
The calculated envelope value is advantageously compared, in a comparator element, with a gain parameter. The latter may correspond to the reciprocal of the maximum possible gain in the hearing aid. Then, if the envelope value falls below the adjustable gain parameter, this means that the signal to be processed lies below the adjusted discomfort threshold; and the gain parameter, instead of the envelope value, is used for the further processing, so that oversensitivity of the signal processing of the hearing aid is prevented.
In an advantageous embodiment, the signal value and the calculated envelope value are compared with one another in a comparator element, in order to prevent the value of the ratio of the signal value and the envelope value from exceeding the representable number range of from xe2x80x9cxe2x88x921xe2x80x9d to xe2x80x9c+1xe2x80x9d.
If the ratio of the signal and envelope values exceeds the value xe2x80x9c1xe2x80x9d in magnitude, an output fixed value is determined by a switch element as the signal to be output, instead of the calculated ratio, so as to comply with the specified number range of from xe2x80x9cxe2x88x921xe2x80x9d to xe2x80x9c+1xe2x80x9d.
In the method according to the invention, constant sound pressure levels are produced by taking the ratio of the signal value and the envelope value of the signal value. The output signal comprised of the individual ratio values is limited with respect to its sound pressure level, and it has a substantially constant envelope curve.
In a method variation according to the invention, the signal value to be output or further processed corresponds to the value of the ratio or the signal value and the envelope value if the envelope value exceeds the signal value. The representable number range of from xe2x80x9cxe2x88x921xe2x80x9d to xe2x80x9c+1xe2x80x9d can hence be complied with in digital signal processing.
If the ratio of the signal value and the envelope value then leaves the representable number range, i.e. its magnitude exceeds the value xe2x80x9c1xe2x80x9d, then not the ratio but rather a predeterminable output fixed value within the representable number range is used as the signal value to be output.
Other method variations have already been described in connection with the hearing aid according to the invention, or will be presented with the aid of the exemplary embodiment.